1. Field of the Invention
The invention relates to a method for measuring the absorption coefficient and reduced scattering coefficient of a medium, more particularly to a method using circularly polarized photon pairs for measuring the absorption coefficient and reduced scattering coefficient of a multiple-scattering medium.
2. Description of the Related Art
Human tissue is a highly scattering medium, and has the characteristics of having a scattering coefficient much greater than the absorption coefficient thereof. A conventional method of obtaining an image in a scattering medium involves measuring of absorption coefficients so as to obtain an image with contrast. However, when it is desired to obtain the image of an object in a multiple scattering medium, since the light waves are highly scattered, the resultant image is blurred, and the resolution of the image is also reduced considerably. Therefore, if the scattering effect of light in a medium can be reduced, or if only slightly scattered snake photons and ballistic photons are selected, together with enhancement of the sensitivity to absorption coefficients, the resolution of an image in the medium can be enhanced. However, this is not suitable for imaging objects in multiple scattering media.
At present, methods for imaging in a multiple scattering medium mainly include time-domain and frequency-domain imaging techniques. The concept of diffused photon density wave (DPDW) is proposed in the frequency-domain technique. DPDW satisfies the diffusion equation, and can be relied upon to obtain definite amounts of the absorption and scattering coefficients of a test object, such as a tumor, thereby permitting recovery of an image of the test object in the scattering medium. The frequency-domain technique is currently more suitable for imaging in a multiple scattering medium, but has the drawback that the spatial resolution is not high. Thus, how to enhance imaging resolution in applications that involve a multiple scattering medium is currently an important topic in the industry.
In co-pending U.S. patent application Ser. No. 10/763,655 entitled “METHOD FOR MEASURING THE ABSORTPION COEFFICIENT AND THE REDUCED SCATTERING COEFFICIENT OF A MULTIPLE SCATTERING MEDIUM”, filed on Jan. 23, 2004, there is disclosed a method for measuring absorption and reduced scattering coefficients of a multiple scattering medium, in which a coherent light beam is outputted. The coherent light beam includes linearly polarized P and S wave components having mutually orthogonal polarizations and frequencies ωP and ωS, respectively. Then, the coherent light beam is split into a signal beam and a reference beam, which include the P and S wave components. The signal beam is subsequently projected into the medium. Optical interference signals of the reference beam and the signal beam penetrating the medium are respectively detected and converted into heterodyne interference electrical signals. Thereafter, the two heterodyne interference electrical signals are compared to obtain is amplitude attenuation and phase delay of the signal beam penetrating the medium, from which the absorption and reduced scattering coefficients of the medium at a position where the signal beam penetrated the medium are inferred.